You keep mentioning your "experience" scouting, but you haven't scouted Ryan Lavarnway, and neither have any of us. We are regurgitating information from actual scouts and what little exposure we've had regarding his abilities.
According to those scouts, he could probably (notice probably) hit Major League pitching right now, but he needs a lot of work defensively, and needs time before he becomes a regular MLB catcher (what the Red Sox view him as, not a DH). What you're doing here is nothing but a bunch of assumptions and leaps of faith, specially given the fact that you're projecting actual counting stats.
As for Salty, he has his shortcomings, but he is at least adequate. That is a claim that cannot be made about Lavarnway until he proves it at the MLB level, and the Red Sox are doing the right thing by bringing him along slowly so he has every opportunity to succeed.